In such filters it is unavoidable that the filtrate will be more unpurified at that side, where the filtering means, usually filtering cloth, dives into the liquid. Later during each revolution the deposited filter layer will serve as filtering medium and the filtrate will be successively cleaner. Thus, the filtrate will have a coarser structure at that side, where the periphery of the rotor moves in a downward direction, and a finer structure at the opposite side of the container. This will have a negative effect on the total efficiency of the filter.
In order to reduce this drawback it has previously been proposed to divide the filtrate in two fractions, a so called pre-filtrate and a so called clear filtrate, the pre-filtrate fraction being derived from that side, where the filtrate has a coarser structure, and the clear filtrate fraction from the opposite side. The pre-filtrate fraction can then for example be returned to the unfiltered liquid suspension.
The present invention relates to filters having this facility and more particularly it relates to an arrangement in filters as described in the preamble, in which a pre-filtrate fraction is produced by discharge means situated at that side of the container, where the periphery of the rotor moves in the downward direction into the liquid for extracting the pre-filtrate fraction from the filtrate in the container at said side, while a clear filtrate fraction is derived from the opposite side of the container, where the periphery of the rotor moves in an upward direction from the liquid surface.
In the previously proposed known filter constructions the discharge means for producing the pre-filtrate fraction are shaped as conventional overflow means having an axial extension substantially corresponding to the length of the rotor.
A drawback for this known solution is that it is difficult to adjust the overflow means such that a desired pre-filtrate fraction is obtained, in particular when the total filtrate flow varies. Thus, if for example the total filtrate flow increases with 10% the pre-filtrate fraction discharged via the overflow means of conventional type can increase with 25-45% of the total filtrate flow, which of course is disadvangeous from an economical point of view. In case of disc filters, the overflow means for the pre-filtrate will be several times longer than the overflow means for the clear filtrate, if the said first overflow means are brought to follow the filtering surfaces on the rotor, which is necessary for achieving a satisfactory function. In this ease a small variation of the total filtrate flow will lead to a drastic variation of the pre-filtrate fraction.